Magnetic leakage flux methods are an important component of quality control, both in the production process and during the cyclically recurring testing of the finished parts, in nondestructive testing in respect of defects of semi-finished product and finished parts. In relation to some bothersome properties of the materials, such as roughness of the surface or scale coating in the case of hot-rolled products, magnetic leakage flux methods are less sensitive than e.g. the eddy current method or ultrasonic testing. As a result, there is a better ratio between used signal and noise signal (S/N ratio), as a result of which a more reliable fault detection is facilitated.
In an apparatus for detecting defects by means of leakage flux measurement, a test volume of the object to be tested is magnetized by means of a magnetizing device and scanned with the aid of at least one magnetic-field-sensitive probe (leakage flux probe) for detecting magnetic leakage fields caused by the defects. In the process, there is a relative movement in one scanning direction between the probe and the surface of the material to be tested. During the scanning, the probe is kept at a relatively small, but finite testing distance from the surface of the material to be tested. An individual probe passes over a testing track, the width of which is determined by the effective width of the probe transversely to the scanning direction.
The magnetic flux or magnetic field generated in the material to be tested by the magnetizing device is distributed substantially homogenously in space in material that is free from faults. In this case, there are also no substantial magnetic field gradients in the regions near the surface. Cracks and other defects, such as e.g. shrink holes, inclusions, or other inhomogeneities such as e.g. welding seams, etc., act as regions of increased magnetic resistance, and so field components in the vicinity of a defect are guided around the defect and pushed out of the metal into the region near the surface. The field components pushed thereout are detected in the leakage flux method for detecting the defects. In the case of a leakage flux measurement, a defect is detectable if the field components pushed out of the test object extend out to the region of the probe and cause a change in the field there which is sufficient for detection.
Depending on how the material to be tested is magnetized, the leakage flux testing methods or testing apparatuses are subdivided into methods or apparatuses with DC field magnetization (DC leakage flux testing) and methods or apparatuses with AC field magnetization (AC leakage flux testing).
When pipes are tested, capturing of both outer faults, i.e. faults or defects on the outer side of the pipe, and inner faults, i.e. faults on the pipe inner side and faults in the pipe wall, is sought after. To this end, use is usually made of methods with DC field magnetization (DC leakage flux testing). Here, a substantial advantage of DC field magnetization is used, specifically the great penetration depth, and so it is also possible to capture inner faults and faults in the pipe wall.
In the methods and apparatuses considered here, use is made of a probe arrangement for carrying out the testing, said probe arrangement having a probe array with a multiplicity of magnetic-field-sensitive probes, which are arranged next to one another in a first direction (width direction). The electrical probe signals, i.e. the electrical signals from the probes, or signals derived therefrom, are evaluated together by means of an evaluating device for qualifying the defects. By using a probe array, the testing width covered during a scanning process may be substantially larger than the testing width covered by an individual probe. Furthermore, the spatial resolution of the width direction is determined by the probe width of the individual probes. By using probe arrays, efficient testing of test objects in a continuous method is rendered possible.
When dimensioning the individual probes in respect of the probe width thereof, there usually is orientation on the basis of the so-called minimum fault length. The minimum fault length is the fault length (or defect length) above which the maximum amplitude of the probe signal, i.e. the highest testing sensitivity, and the maximum reproducibility are achieved. In the relevant standards, probe widths of 30 mm, or of one, or half a, minimum fault length, are specified, wherein the minimum fault length may be e.g. 25 mm or 50 mm, depending on standard. As a result of the reference to the minimum fault length, it is possible to obtain a good compromise between a number of probes which is as small as possible with, at the same time, a probe array which is as long or wide as possible (cost optimization) and the maximum admissible probe width (generally half the minimum fault length) considered to be required for a good reproducibility of the defect detection.
There is therefore needed a method and an apparatus for leakage flux testing, by means of probe arrays, of ferromagnetic material to be tested, which facilitate reliable testing in respect of faults of different types of fault.